This invention relates to a ceramic-metal composite suitable for use in a high-temperature environment. More specifically, this invention relates to a ceramic-metal composite electrode for use as a current collector in the channel of a magnetohydrodynamic generator.
The principle of magnetohydrodynamic power generation utilizes heat to produce a high-velocity stream of electrically conducting fluid, which is then passed through a magnetic field to convert the kinetic energy of the stream into electrical energy. A typical diagonal window frame MHD power generator is an elongated duct or channel constructed of a large number of open rectangular forms or "window frames" fastened together side-by-side and insulated from each other. Around the inner perimeter of each frame are located a number of individual, generally rectangular, electrodes for collecting the electrical energy generated in each frame by the passage of the high-temperature conductive fluid. Other generator geometries can also be used but in each case a number of electrodes are present and separated from each other by an electrical insulator since some will act as anodes and some as cathodes as the plasma passes through the channel.
The plasma within the channel may reach temperatures up to 2800.degree. C., resulting in electrode-plasma temperatures up to 2100.degree. C. The plasma may be an ionized gas or an inert gas seeded with a conductor such as potassium. The plasma passes through the duct at a rate of speed which may approach and even exceed the sonic velocity. The plasma environment may be slightly oxidizing, depending upon the particular fluid being used and its source. Thus, it is a problem to find a material from which electrodes can be made which can withstand the rigors of such an environment. Ideally, such an electrode material would be a highly refractory ceramic that is highly corrosion-erosion-resistant, dense, strong, thermal-shock-resistant, chemically inert and a good electrical conductor.
Electrodes have been fabricated from a variety of materials like zirconia, either alone or combined with a metal alloy such as Inconel. They have been tried in MHD generators but do not possess all of the desirable characteristics required of a good high-temperature electrode.